Ambient-air vaporizers are used to vaporize cryogenic liquids before being piped to downstream operations or being released to the surrounding atmosphere. Such vaporizers are typically of the Starfin-type and have longitudinally finned pipe lengths arranged in a 3.times.4 rectangular array. A side view of such a vaporizer is shown in FIG. 1 labeled prior art, and a top view is shown in FIG. 2, also labeled prior art.
The liquid to be vaporized enters the fin section Labeled 1 and is passed through the fin pipings in numerical order as depicted in FIG. 2 until it exits from fin pipe 12. The liquid enters the bottom of the first finned pipe, passes through, exits the top and subsequently enters the top of the next pipe where it flows to the bottom, exits, and enters the bottom of the next pipe, etc.
If gas delivery temperature from the vaporizer falls below a preset value, a low temperature shut off valve closes to prevent liquid carry-over to the customer or embrittlement of downstream piping caused by extreme cold temperatures. For example, if carbon-steel piping is used to transport the gas downstream of the vaporizer, a shut off temperature of about -23.degree. C. is employed to prevent embrittlement of the piping. In extremely cold weather conditions and/or high product flow, the vaporizers can develop ice formations and freeze-over between the fins occurs. When such a freeze-over occurs, the capacity of the vaporizer decreases, and can thereby cause the gas delivery temperature to fall below such a preset value. This decrease in capacity is due in part to reduced convective heat transfer and ice bridging between fin pipings which allows conduction between warmer fins and colder fins. When such a decrease in capacity occurs it is necessary to physically chop away the ice which is formed between the fin pipings of the vaporizer in order to get the system back on-stream following a shutdown.
Solutions to the problem of reduced capacity due to reduced heat transfer and ice bridging between the fins have included installing low temperature-ductile metal systems downstream which tolerate lower delivery gas temperatures or installing heated vaporizers which use steam or electric heat to vaporize the cryogenic liquid. Additionally, it has been suggested to increase the number of ambient vaporizers in series or parallel connected pairs, thereby enlarging the vaporizing system itself. All of the above solutions require a considerable increase in equipment and process cost.